SPL: Move the omap SPL framework to common/spl

Add a new flag, CONFIG_SPL_FRAMEWORK to opt into the common/spl SPL
framework, enable on all of the previously using boards.  We move the
spl_ymodem.c portion to common/ and spl_mmc.c to drivers/mmc/.  We leave
the NAND one in-place as we plan to replace it later in this series.

We use common/spl to avoid linker problems with respect to merging
constant strings in objects.   Otherwise all strings in common/ will be
linked in and kept which grows SPL in size too much.

Signed-off-by: Tom Rini <trini@ti.com>